BEFORE — What people assume Excel does

Excel doesn't recalculate the whole spreadsheet when you change a cell, but a lot of people assume it does. It actually walks a hidden dependency graph and only recomputes what exactly changed. So here's what's going on under the hood. Every formula cell in Excel keeps a list of the cells that depend on it. So when you change one number, Excel marks all the downstream cells as dirty, follows that chain as far as it needs to go, and recomputes only those cells. And in the right order as well. So if you input before the formulas that need them, and a topological sort as well that runs quietly every time you press enter. And this is why a sheet with a million cells can update in milliseconds. And Excel's been doing this since the 90s. Multi-threaded calculation across independent branches has gotten added later and things like smart recalc for volatile functions like now. But the part that I think is really cool is Excel is the widely used incremental computation engine on the planet, but no one ever really talks about it that way. So just really think it's, you know, one of the most impressive incremental computation engines ever shipped to a billion people.